The invention relates to an exposure device, adapted for use in an image forming apparatus, for scanning a light beam modulated according to image data across an scanning object in order to form an electrostatic latent image on the scanning object.
Electrophotographic image forming apparatus such as copiers or printers are provided with an exposure device. The exposure device scans a light beam, such as a laser beam, modulated according to image data across an scanning object, thereby forming an electrostatic latent image on the scanning object. Electrophotographic image forming apparatus use a developing device to develop the electrostatic latent image into a developer image, and uses an intermediate transfer belt to transfer the developer image on a recording medium. Thus, the apparatus forms an image on the recording medium.
Such apparatus generally use a laser-beam exposure device provided with features of high-speed laser-beam modulation, and therefore, of high-speed scanning.
Along with proliferation of color image forming apparatus in recent years, there has been a growing demand for smaller apparatus provided with higher image forming speed and improved performance in image formation with respect to various kinds of recording media such as thick paper, thin paper, or recycled paper.
This is why recent color image forming apparatus have a tandem configuration where a plurality of image forming sections are provided in alignment each of which performs an electrophotographic image forming process with respect to a corresponding, different color. An image of developer of corresponding color is formed on a scanning object provided in each image forming section. The developer image is transferred from the object to an intermediate transfer belt, and then transferred from the belt to a sheet of recording medium (hereinafter referred to merely as a “sheet”). Since developer images of different colors are first accumulated on the belt, the tandem image forming apparatus forms stable images on any kind of sheet.
In the tandem apparatus, an exposure device is positioned below the image forming sections in view of reducing the physical size, and enhancing usability, of the apparatus. The exposure device emits light beams to the scanning objects positioned above, through windows provided in a housing each including a transmissive member such as a glass plate.
The position below the image forming sections causes the transmissive members to be contaminated with dust such as developer particles fallen from the image forming sections. The contamination of the transmissive member causes improper scanning of the scanning objects. Dust is more likely to fall during a period such as when developing devices are operating in an image forming process, or when the developing devices are replaced.
A solution to the foregoing problems is a color image forming apparatus such as proposed in JP 2002-148910A. The proposed apparatus includes a shutter for shielding a transmissive member (a sealing glass), that is opened to allow a light beam for scanning a scanning object to pass through the transmissive member. At all times other than in image forming process, the shutter is closed to shield the transmissive member in order to prevent contamination thereof.
Even when the shutter is closed, however, dust becomes accumulated on and around the shutter during a period such as when developing devices are being replaced or when the apparatus is on standby.
Also, the image forming sections start or stop image forming process in the order of alignment from upstream to downstream along a direction in which the intermediate transfer belt travels. In other words, a shutter is closed in an upstream one of image forming section while a downstream image forming section is performing image forming process. Thus, dust falling from the downstream section tends to accumulate on the upstream shutter in the closed state.
When the upstream shutter is opened to unshield the transmissive member, the accumulation of dust may fall from the shutter to contaminate the transmissive member.
In light of the foregoing, a feature of the invention is to provide an exposure device that prevents contamination of a transmissive member with accumulation of dust, such as developer particles, on a shutter in the opening action of the shutter, thereby ensuring proper scanning of a scanning object.